In a typical radio receiver, an antenna transduces airborne radio frequency signals of interest into electrical radio frequency signals. These radio frequency signals are reduced to a lower frequency signal in an IF stage, and then demodulated to extract the information of interest. Such information may be in analog or digitized form. Demodulated analog information will typically be provided to an audio processing circuit, and digitized data will be decoded and acted upon by an appropriate logic circuit.
When a broadcast signal first appears on a monitored channel, or when initially tuning a radio receiver to a new channel then having a broadcast signal present thereon, a voltage step typically occurs at the output of the demodulator (in a frequency modulation system). The magnitude of this voltage step may depend upon a variety of factors, but often comprises a netting error that depends particularly upon the frequency tolerance of the transmitting signal oscillator and the tolerance of the local oscillator (or oscillators) in the receiver.
Not infrequently, the magnitude of this voltage step can be the equivalent to 2 kHz of frequency error or more. This constitutes a significant error, since modulation of the desired signal may be limited to a maximum deviation of only 5 kHz. In addition to the error considerations noted above, the voltage step transiently present at the output of the demodulator, in combination with the signal modulation, may saturate any subsequent amplification stages. This also presents a serious impediment to satisfactory reception performance.
These problems are typically transitory in nature, and hence may be acceptable under some operating circumstances. In other situations, however, these problems are unacceptable. For instance, when receiving low deviation FSK data (having, for instance, 500 to 1000 Hz deviation), the demodulated waveform must be recentered to account for any netting error in order to assure that the "1's" and "0's" comprising the digitized data are accurately distinguished. Many prior art radios use diode-RC networks or center slicers to perform this function. Unfortunately, such prior art techniques require up to hundreds of milliseconds to effectively center the demodulated signal For many systems, this means that much important data will be lost.
There exists a need for a device to correct such netting error transients in a relatively short period of time to prevent such loss of information. Such a device should correct netting error transients that occur both when tuning to a new channel, and also when a monitored previously quiescent channel suddenly becomes active.